In a wireless communication system, a so-called measuring node performs one or more positioning measurements that are used to determine the position of a target device. A measuring node may be the target device itself, a separate radio node (i.e., a standalone node), a serving and/or neighboring node of the target device, etc. A measuring node may perform a positioning measurement on one or more downlink signals and/or one or more uplink signals. For example, in at least some scenarios, such as where the target device's serving base station functions as a measuring node, the measuring node may perform a positioning measurement on a downlink signal that the measuring node transmits to the target device and/or on an uplink signal that the measuring node receives from the target device.
Consider for instance the enhanced cell identity (E-CID) positioning method in Long Term Evolution (LTE) systems. This method determines the target device's position using at least the cell ID of a serving and/or a neighboring cell of the target device, as well as at least one positioning measurement. Various types of positioning measurements can be used for E-CID, including for instance power measurements, signal quality measurements, pathloss measurements, angle of arrival measurements, and/or timing measurements. For example, to perform a timing measurement referred to as an “eNB Rx−Tx” time difference measurement, the base station controlling the target device's serving cell 77 functions as the measuring node by measuring the difference between (i) the time when the serving cell sends a downlink signal to the target device; and (ii) the time when the serving cell thereafter receives an uplink signal from the target device. Similarly, to perform a timing measurement referred to as a “UE Rx-TX” time difference measurement, the target device itself functions as the measuring node by measuring the difference between (i) the time when the target device sends an uplink signal to the serving cell; and (ii) the time when the target device thereafter receives a downlink signal from the serving cell.
Regardless of the particular type of positioning measurement(s) performed, measurement(s) performed on a downlink signal are performed on a reference signal (e.g., a cell-specific reference signal, CRS, in LTE), a synchronization signal, a pilot channel, or any other known radio signal. Downlink radio signals transmitted from different cells in this context are distinguished from one another by the different cells' identities. In LTE, for instance, downlink radio signals transmitted from different cells are transmitted based on respective physical cell identities (PCIs). This means that positioning measurements performed on downlink signals from different cells are likewise distinguished from one another by the cell identities respectively associated with those measurements. For example, in approaches where a positioning node (e.g., E-SMLC) determines the target device's position rather than the target device itself determining its position, the target device reports the results of a positioning measurement performed on a downlink signal by indicating the identity of the cell on which that measurement was performed.
Shared cells introduce complexities to positioning. A shared cell is a type of downlink (DL) coordinated multi-point (CoMP) where multiple geographically separated transmission points (TPs) dynamically coordinate their downlink transmissions. For example, a shared cell may include low power radio resource heads (RRHs) within a macro cell's coverage, where the transmission/reception points created by the RRHs have the same cell IDs as that of the macro cell. A TP may comprise one or more antenna ports, and TP identification may in some examples comprise an identification of its antenna or antenna port. Regardless, the unique feature of a shared cell (at least in an LTE context) is that all TPs within the shared cell have the same physical cell ID (PCI). This, coupled with tight synchronization in terms of transmission timings between the TPs within a shared cell, enables the physical signals and channels transmitted from the TPs to be combined over the air. This combining increases the average received signal strength, leading to improved coverage of synchronization and control channels.